1. Field of the Invention
The present invention relates to a liquid injection type screw compressor, and more particularly, to a liquid injection type screw compressor having a male rotor shaped to reduce a liquid compression phenomena.
2. Description of the Prior Art
Referring to FIG. 11, a conventional liquid injection type screw compressor has a casing 12 that includes an intake port at a first end and a discharge port 11 at an opposing second end in the longitudinal direction. The first end serves as an intake end of the casing 12 and the second end serves as a discharge end of the casing 12. A male rotor 13 and a female rotor 14, both having helical teeth, are installed in casing 12 with their helical teeth in engagement during rotation of rotors 13 and 14. A rotor tooth space 15 exists between rotors 13 and 14.
In a screw compressor as described above, gas compression begins when an intake process introduces gas from the intake port of casing 12 into rotor tooth space 15. Compression reduces the volume of rotor tooth space 15 during rotation of rotors 13 and 14, thereby compressing the gas. The compressed gas is discharged through discharge port 11 of casing 12.
The discharge process calls for conveying compressed gas contained in rotor tooth space 15 to discharge port 11 from the time when rotor tooth space 15 couples with the discharge port 11 until the rotation of rotors 13 and 14 reduces the volume of rotor tooth space 15 to zero. The discharge process comprises three stages, shown in FIGS. 11 through 13, characterized by the form of a discharge path coupling rotor tooth space 15 to discharge port 11.
Referring to FIG. 11, in the first stage, the compressed gas is discharged in the directions of both a radius and an axis of rotor tooth space 15.
Referring to FIG. 12, in the second stage, the compressed gas is discharged only in the axial direction of rotor tooth space 15 because a tooth of male rotor 13 engages a tooth of female rotor 14, thereby sealing off the radial path of discharge. This stage of the discharge process is called the "semi-closed condition."
Referring to FIG. 13, in the third stage, there is no discharge path connecting rotor tooth space 15 to discharge port 11. This stage is known as the "completely closed condition."
The surface of each tooth of rotors 13 and 14 is thoroughly lubricated by a liquid, such as oil, injected into casing 12 in order to absorb heat generated during gas compression and to effect a seal between the rotors 13 and 14, and between the rotors 13 and 14 and the casing 12. The seal formed by the oil reduces leakage of compressed gas from the discharge port 11 to the intake port.
The conventional liquid injection type screw compressor described above has no path connecting rotor tooth space 15 to discharge port 11 during the third stage of the discharge process when the compressor is in the completely closed condition. While this condition exists, the volume of rotor tooth space 15 continues to decrease. Thus, the lubricating liquid is trapped in rotor tooth space 15 and has pressure applied upon it by the rotors 13 and 14, causing a sudden radical increase of pressure upon the rotors. This increase in pressure is generally called a "liquid compression phenomenon."
The pressure increase caused by this liquid compression phenomenon imposes a pulsed load upon the rotors 13 and 14 and their respective bearings. This pulsed load reduces the life span of the bearings and creates undesirable vibration when the compressor is in operation.
Additionally, as the rotation speed of rotors 13 and 14 is increased, the flow resistance of the liquid against the surface of the teeth also increases. Thus, a form of the liquid compression phenomenon also occurs during the second stage of the discharge process, when the path is only partly closed and a semi-closed condition exists. Especially in cases where the compressed gas consists of light gas, such as hydrogen gas and helium gas, liquid tends to be trapped in the rotor tooth space 15 during the discharge process when the path is half or completely closed.
In order to reduce the above liquid compression phenomenon, various modifications are made to the shape of discharge port 11 of casing 12 as well as to the shape of the ends of rotors 13 and 14 facing discharge port 11. However, none of these modifications are sufficiently effective, each suffering from various drawbacks. The various drawbacks include a considerable quantity of leakage of compressed gas from the discharge port 11 to the intake port, and a resultant substantial decrease of compression efficiency.
Referring to FIG. 14, a rotor is shown that is modified so as to prevent the occurrence of the liquid compression phenomenon. A recess 16 on the discharge end of male rotor 13 is made by forming a step on the surface of each rotor tooth. Recess 16 is formed by cutting the discharge end of male rotor 13. This structure, however, requires the initiation of the pressure relieving of the liquid and gas even before the liquid compression phenomenon occurs in order to effect a complete elimination of the liquid compression phenomenon. Thus, this structure reduces the compression efficiency of the unit.
The male rotor 13 in FIG. 14, viewed without the obstruction of the female rotor 14, has a stepped recess formed by two slanted planes 18 and 19, cut into the discharge end of the rotor. Plane 18 runs parallel to the rotor axis and plane 19 runs such that its projection intersects the rotor axis. To prevent a run-off from being formed before an initiation of closing it is necessary to position the recess inward of an initiation line of closing. Enclosing portion 17, between rotors 13 and 14, is narrower closer to the root than it is further from the root at the time of the initiation of closing.
Referring to FIG. 15, in the above described embodiment, further rotation of the rotors after the initiation of the closing causes the closing line to reach the position represented by broken line "a". The path for relieving the liquid is part "b" which is represented by slanting lines. It is clear that only a small opening is available for relieving the liquid immediately after the initiation of closing.
Referring to FIG. 16, the release of liquid in the axial direction is impeded because of step portion 16a while the liquid is released unimpeded in the circumferential direction. Thus, the above conventional configuration is not capable of preventing liquid compression completely, Instead, it produces a pressure increase at the beginning of closing.
In order to completely eliminate liquid compression, it is necessary to cut a recess extending past the closing initiation line. This, however, results in an excessive leakage of the compressed gas. Furthermore, referring to FIGS. 17 and 18, at high peripheral velocity of the rotor, the liquid on the surface of the casing 12 flows in direction "d", which is the reverse of direction "c" in which the enclosed liquid tends to run off along the circumference. The opposing flows thus resist the relief of the liquid in the circumferential direction. Therefore, it is necessary to cut a recess that is larger than the closing initiation line to completely eliminate liquid compression.